feat: add observability improvements (metrics, audit log, structured logging)

- Add Prometheus metrics package (internal/metrics) with deployment,
  container health, webhook, HTTP request, and audit counters/histograms
- Add audit_log SQLite table via migration 007
- Add AuditEntry model with CRUD operations and query methods
- Add audit service (internal/service/audit) for recording user actions
- Instrument deploy service with deployment duration, count, and
  in-flight metrics; container health gauge updates on deploy completion
- Instrument webhook service with event counters by app/type/matched
- Instrument HTTP middleware with request count, duration, and response
  size metrics; also log response bytes in structured request logs
- Add audit logging to all key handler operations: login/logout, app
  CRUD, deploy, cancel, rollback, restart/stop/start, webhook receipt,
  and initial setup
- Add GET /api/audit endpoint for querying recent audit entries
- Make /metrics endpoint always available (optionally auth-protected)
- Add comprehensive tests for metrics, audit model, and audit service
- Update existing test infrastructure with metrics and audit dependencies
- Update README with Observability section documenting all metrics,
  audit log, and structured logging

internal/models/models_test.go

@@ -23,6 +23,7 @@ const (
 	testBranch    = "main"
 	testValue     = "value"
 	testEventType = "push"
+	testAdmin     = "admin"
 )
 
 func setupTestDB(t *testing.T) (*database.Database, func()) {
@@ -183,7 +184,7 @@ func TestUserExists(t *testing.T) {
 		defer cleanup()
 
 		user := models.NewUser(testDB)
-		user.Username = "admin"
+		user.Username = testAdmin
 		user.PasswordHash = testHash
 
 		err := user.Save(context.Background())
@@ -781,6 +782,179 @@ func TestCascadeDelete(t *testing.T) {
 	})
 }
 
+// AuditEntry Tests.
+
+func TestAuditEntryCreateAndFind(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	entry := models.NewAuditEntry(testDB)
+	entry.Username = testAdmin
+	entry.Action = models.AuditActionLogin
+	entry.ResourceType = models.AuditResourceSession
+
+	err := entry.Save(context.Background())
+	require.NoError(t, err)
+	assert.NotZero(t, entry.ID)
+
+	entries, err := models.FindAuditEntries(context.Background(), testDB, 10)
+	require.NoError(t, err)
+	require.Len(t, entries, 1)
+	assert.Equal(t, testAdmin, entries[0].Username)
+	assert.Equal(t, models.AuditActionLogin, entries[0].Action)
+	assert.Equal(t, models.AuditResourceSession, entries[0].ResourceType)
+}
+
+func TestAuditEntryWithAllFields(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	entry := models.NewAuditEntry(testDB)
+	entry.UserID = sql.NullInt64{Int64: 1, Valid: true}
+	entry.Username = testAdmin
+	entry.Action = models.AuditActionAppCreate
+	entry.ResourceType = models.AuditResourceApp
+	entry.ResourceID = sql.NullString{String: "app-123", Valid: true}
+	entry.Detail = sql.NullString{String: "created new app", Valid: true}
+	entry.RemoteIP = sql.NullString{String: "192.168.1.1", Valid: true}
+
+	err := entry.Save(context.Background())
+	require.NoError(t, err)
+
+	entries, err := models.FindAuditEntries(context.Background(), testDB, 10)
+	require.NoError(t, err)
+	require.Len(t, entries, 1)
+	assert.Equal(t, int64(1), entries[0].UserID.Int64)
+	assert.Equal(t, "app-123", entries[0].ResourceID.String)
+	assert.Equal(t, "created new app", entries[0].Detail.String)
+	assert.Equal(t, "192.168.1.1", entries[0].RemoteIP.String)
+}
+
+func TestAuditEntryFindByResource(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	// Create entries for different resources.
+	for _, action := range []models.AuditAction{
+		models.AuditActionAppCreate,
+		models.AuditActionAppUpdate,
+		models.AuditActionAppDeploy,
+	} {
+		entry := models.NewAuditEntry(testDB)
+		entry.Username = testAdmin
+		entry.Action = action
+		entry.ResourceType = models.AuditResourceApp
+		entry.ResourceID = sql.NullString{String: "app-1", Valid: true}
+
+		err := entry.Save(context.Background())
+		require.NoError(t, err)
+	}
+
+	// Create entry for a different resource.
+	entry := models.NewAuditEntry(testDB)
+	entry.Username = testAdmin
+	entry.Action = models.AuditActionLogin
+	entry.ResourceType = models.AuditResourceSession
+
+	err := entry.Save(context.Background())
+	require.NoError(t, err)
+
+	// Find by resource.
+	appEntries, err := models.FindAuditEntriesByResource(
+		context.Background(), testDB,
+		models.AuditResourceApp, "app-1", 10,
+	)
+	require.NoError(t, err)
+	assert.Len(t, appEntries, 3)
+
+	// All entries.
+	allEntries, err := models.FindAuditEntries(context.Background(), testDB, 10)
+	require.NoError(t, err)
+	assert.Len(t, allEntries, 4)
+}
+
+func TestAuditEntryCount(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	count, err := models.CountAuditEntries(context.Background(), testDB)
+	require.NoError(t, err)
+	assert.Equal(t, 0, count)
+
+	entry := models.NewAuditEntry(testDB)
+	entry.Username = testAdmin
+	entry.Action = models.AuditActionLogin
+	entry.ResourceType = models.AuditResourceSession
+
+	err = entry.Save(context.Background())
+	require.NoError(t, err)
+
+	count, err = models.CountAuditEntries(context.Background(), testDB)
+	require.NoError(t, err)
+	assert.Equal(t, 1, count)
+}
+
+func TestAuditEntryFindLimit(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	for range 5 {
+		entry := models.NewAuditEntry(testDB)
+		entry.Username = testAdmin
+		entry.Action = models.AuditActionLogin
+		entry.ResourceType = models.AuditResourceSession
+
+		err := entry.Save(context.Background())
+		require.NoError(t, err)
+	}
+
+	entries, err := models.FindAuditEntries(context.Background(), testDB, 3)
+	require.NoError(t, err)
+	assert.Len(t, entries, 3)
+}
+
+func TestAuditEntryOrderByCreatedAtDesc(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	actions := []models.AuditAction{
+		models.AuditActionLogin,
+		models.AuditActionAppCreate,
+		models.AuditActionLogout,
+	}
+
+	for _, action := range actions {
+		entry := models.NewAuditEntry(testDB)
+		entry.Username = testAdmin
+		entry.Action = action
+		entry.ResourceType = models.AuditResourceSession
+
+		err := entry.Save(context.Background())
+		require.NoError(t, err)
+	}
+
+	entries, err := models.FindAuditEntries(context.Background(), testDB, 10)
+	require.NoError(t, err)
+	require.Len(t, entries, 3)
+
+	// Newest first (logout was last inserted).
+	assert.Equal(t, models.AuditActionLogout, entries[0].Action)
+	assert.Equal(t, models.AuditActionAppCreate, entries[1].Action)
+	assert.Equal(t, models.AuditActionLogin, entries[2].Action)
+}
+
 // Helper function to create a test app.
 func createTestApp(t *testing.T, testDB *database.Database) *models.App {
 	t.Helper()